In recent years, demands for more and more improved density and integration of various electric devices have been called for in the field of the production of semiconductor and the like devices which require microprocessing. Along with such demands, strictness of the performance required for photography techniques has been in the extreme, in order to realize miniaturization of resist patterns. Increment of the resolving power of photoresists and shortening of the wave length of exposure lights are equally responsible for carrying out the miniaturization techniques.
In general, resolution (Res) of the optical system can be expressed by the Rayleigh's equation, namely Res=k.multidot..gamma./NA (wherein k is a process factor, .gamma. is a wave length of exposure light source and NA is a numerical aperture of lens). It can be understood that shortening of the wave length at the time of exposure is effective in resolving a minute pattern (namely in obtaining high resolution) by reducing the rendering line width. Actually, with the reduction of minimum rendering line width, the exposure wave length has been shifted to the g-line (436 nm) and then to the i-line (365 nm) of high pressure mercury lamp, and production of a device in which KrF eximer laser (248 nm) has also been examined. In addition, application of an eximer laser having more shorter wave length, particularly ArF (193 nm), is regarded as a bright prospect of effecting further minute processing.
With regard to photoresists to be exposed to a short wave length light, improvement of integration capacity has been examined on not the monolayer resists which have been produced in the prior art but a multilayer resist system of two or more layers to which surface lithography was applied. However, the complicated process which has been deterring the multilayer resist from its realization is still problematic.
In addition, in the case of the eximer lasers including KrF eximer laser, it is considered in general that it is necessary to improve cost performance of the laser, because not only the gas life is short but also the exposure apparatus itself is expensive to begin with.
Developed in response to this necessity is the so-called chemical amplification type resist which became the main stream from the use for KrF eximer laser exposure. The chemical amplification type resist has a mechanism in which an acid is generated by exposure from a photo acid generator which is present in a catalytically effective amount in the system, and a protecting group of an alkali-soluble group of a binder or a low molecular weight compound is catalytically eliminated by the thus generated catalytically effective amount of acid, thereby ensuring discrimination of solubility in an alkaline developing solution. Since the chemical amplification type resist catalytically uses an acid generated by an optical reaction, its high sensitivity is expected.
In general, the chemical amplification type resist can be divided roughly into three types, namely a 2 component system, a 2.5 component system and a 3 component system in common names. The 2 component system is a combination of a photo acid generator with a binder resin. Said binder resin is a resin having in its molecule a group which is decomposed by the action of an acid and thereby increase solubility of the resin in an alkaline developing solution (also to be referred to as acid decomposable group hereinafter). The 2.5 component system is a modification of the 2 component system which further contains a low molecular weight compound having an acid decomposable group. The 3 component system contains a photo acid generator, an alkali-soluble resin and the above-described low molecular weight compound.
However, shortened wave length of the exposure light has caused a new problem. That is, in a photoresist, a raw material having excellent transparency for a light of short wave length has a problem in terms of dry etching resistance. On the contrary, a raw material having excellent dry etching resistance has a problem of poor transparency. Compatibility of dry etching resistance with transparency is basically an issue of the performance of binder resin in the photoresist layer.
Examples of the binder include a novolak resin and poly-p-hydroxystyrene. The novolak resins widely used as an alkali-soluble resin for i-line resists, and poly-p-hydroxystyrene is used as a base polymer for KrF eximer laser resists. These binders do not cause serious problems when a light of long wave length is used. However, it is not the same in the case of a short wave length light. Particularly, since the aforementioned resins have a high optical density within the wavelength region of 170 nm to 220 nm, it is virtually impossible to use them directly in the conventional manner. Because of this, great concern has been directed toward the development of a resin which has high light transparency and high dry etching resistance.
As one of the general answers for this issue, there is a method in which an alicyclic hydrocarbon moiety is introduced into a resin. Also, there is a method in which the naphthalene nucleus of an aromatic compound is utilized. Particularly, there are a number of reported methods in which both of the requirements for light transparency and dry etching resistance are gratified by the introduction of an alicyclic hydrocarbon moiety. An example of such methods is described in Journal of Photopolymer Science and Technology, vol.3, p.439, 1992.
However, the resist for KrF eximer laser use and the resist for use in the exposure to far ultraviolet rays having a wavelength region of 170 nm to 220 nm are completely different from each other because of the different exposure wavelength regions, and it is the present situation that there is no distinct guiding principle on how to design such a resist composition for far ultraviolet ray exposure use.
On the other hand, it is important to select a group as the acid decomposable group in an acid decomposable group-containing resin, because it exerts influences particularly upon sensitivity and resolution of resists and even upon their aging stability.
As the acid decomposable group which protects carboxylic acid groups, mainly t-butyl ester and the like tertiary alkyl esters and tetrahydropyranyl ester, ethoxyethyl ester and the like acetal esters have so far been reported. However, the t-butyl ester group has a problem in that elimination capacity by the generated acid is poor and the sensitivity therefore is reduced. On the other hand, the tetrahydropyranyl ester, ethoxyethyl ester and the like acetal esters have a serious problem in terms of aging stability, because they are apt to be decomposed even at ordinary temperature.
In addition, JP-A-5-346668 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes that 3-oxocyclohexyl ester group be used as the acid decomposable group, but this is not always satisfactory in terms of sensitivity.
Thus, it is not necessarily clear about how to design a carboxylic acid-protecting acid decomposable group which satisfies both sensitivity and aging preservation stability in a photoresist.